Retention elements such as posts are often used to mount a device onto a mounting surface. As an example, electronic component sockets may have one or more mounting posts depending from the socket body, these posts being cooperative with associated cavities in a mounting surface such as a circuit board to retain the socket on the board.
Tight dimensional tolerances are required of cylindrical retention elements engageable with cooperating cavities to ensure adequate retention force acting on the retention element without causing damage to the element on insertion. Extreme variance from desired dimensions can prevent insertion or eliminate the retention capability.
In order to ease tolerance restrictions, a retention element having a wedge-shaped groove disposed axially along the length of the element has been used. Typically, the groove has an angular width of about 30.degree., and a depth greater than the radius of the retention element. It has been established that such a geometry enables insertion of a retention element of diameter slightly larger than or equal to that of a cooperating cavity. In prior art retention elements having a groove as described, a deformation of the retention element cross-section is caused by limited flexing of the element sides inwardly during insertion of the element into a cavity. Rather than retaining a roughly circular cross-section, the width necessarily decreases as the sides compress. Further, the length of the cross-section as measured from the opening of the groove to the side of the element opposite the groove vertex increases. It is this deformation from a circular to an oblong cross-section which causes increased insertion forces and skiving when the prior art element is inserted into a cavity.
Several problems are prevalent with this prior art configuration. First, a minimum amount of retention element material must remain in order to resist shear and withdrawal forces. This in turn limits the maximum size of the wedge-shaped groove, and thus the amount by which the element can compress. As a result, it has been found that a retention element slightly larger than a corresponding cavity can suffer skiving wherein material is shaved off the element as it is forced into the cavity. The skiving of material and resultant decrease in retention element diameter necessarily reduces retention forces developed between the prior art retention element and the cavity during successive insertion and withdrawal cycles.
A second problem with the prior art configuration involves a disparity between an amount of force required to insert the retention element into its cavity and a force required to extract it, or equivalently, a retention force existing between the retention element and the cavity. It would be desirable to have a smaller ratio of withdrawal force to insertion force as compared to that of the prior art. With such a smaller ratio, a relatively small insertion force would be required to achieve a significant retention force between retention element and cavity.